Occupational asthma

The true frequency of occupational asthma is not known, but underreporting is likely. Published reports, which come from surveillance schemes, compensation registries or epidemiological studies, estimate that occupational asthma may account for about 9–15% of adult onset asthma. ^858-860

The diagnosis should be suspected in all adults with symptoms of airflow limitation, and positively searched for in those with high-risk occupations or exposures. Patients with pre-existing asthma aggravated non-specifically by dust and fumes at work (work-aggravated asthma) should be distinguished from those with pre-existing asthma who become additionally sensitised to an occupational agent.

Several hundred agents have been reported to cause occupational asthma and new causes are reported regularly in the medical literature.

Occupational asthma should be considered in all workers with symptoms of airflow limitation (see Annex 10). The best screening question to ask is whether symptoms improve on days away from work. This is more sensitive than asking whether symptoms are worse at work, as many symptoms deteriorate in the hours after work or during sleep. The use of non-leading questions is advocated.⁸⁸⁰

Adults with suspected asthma or unexplained airways obstruction should
be asked:

• Are you the same, better, or worse on days away from work?
• Are you the same, better, or worse on holiday?

Those with positive answers should be investigated for occupational asthma.

[BTS/SIGN 2019]

Occupational asthma can be present when tests of lung function are normal, limiting their use as a screening tool. Asthmatic symptoms improving away from work can produce false negative diagnoses, so further validation is needed.

Although skin-prick tests or blood tests for specific IgE are available, there are few standardised allergens commercially available which limits their use. A positive test denotes sensitisation, which can occur with or without disease. The diagnosis of occupational asthma can usually be made without specific bronchial provocation testing, considered to be the gold standard diagnostic test. The availability of centres with expertise and facilities for specific provocation testing is very limited in the UK and the test itself is time consuming.

There are several validated methods for interpreting serial PEF records for a diagnosis of occupational asthma which differ in their minimal data requirements.

The original discriminant analysis method requires:

• at least three days in each consecutive work period
• at least four evenly spaced readings per day
• at least three series of consecutive days at work with three periods away from work (usually about three weeks).⁸⁹²

Shorter records without the requirement for three consecutive days at work can be analysed using the area between curves score. This requires at least eight readings a day on eight work days and three rest days.⁸⁹³ A statistical method using the addition of timepoint analysis requires the waking time to be similar on rest and work days.⁸⁹⁴

The analysis is best done with the aid of a criterion-based expert system. Suitable record forms and support are available from www.occupationalasthma.com

The aim of management is to identify the cause, remove the worker from exposure, and for the worker to have worthwhile employment.

Several studies have shown that the prognosis for workers with occupational asthma is worse for those who remain exposed for more than one year after symptoms develop, compared with those removed earlier.^911-913

1. 858. Meredith S, Nordman H. Occupational asthma: measures of frequency from four countries. Thorax 1996;51(4):435-40.
2. 859. Blanc PD, Toren K. How much adult asthma can be attributed to occupational factors? Am J Med 1999;107(6):580-7.
3. 860. Balmes J, Becklake M, Blanc P, Henneberger P, Kreiss K, Mapp C, et al. American Thoracic Society Statement: Occupational contribution to the burden of airway disease. Am J Respir Crit Care Med 2003;167(5):787-97.
4. 880. Fishwick D, Barber CM, Bradshaw LM, Harris-Roberts J, Francis M, Naylor S, et al. Standards of care for occupational asthma. Thorax 2008;63(3):240-50.
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6. 893. Moore VC, Jaakkola MS, Burge CB, Pantin CF, Robertson AS, Vellore AD, et al. PEF analysis requiring shorter records for occupational asthma diagnosis. Occupational Medicine (Oxford) 2009;59(6):413-7.
7. 894. Burge CBSG, Moore VC, Pantin CFA, Robertson AS, Burge PS. Diagnosis of occupational asthma from time point differences in serial
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10. 913. Gannon PF, Weir DC, Robertson AS, Burge PS. Health, employment, and financial outcomes in workers with occupational asthma. Brit J Ind Med 1993;50(6):491-6.